I would say Pyra (Blogger), Picasa, Keyhole (Google Earth), Urchin / Measure Map (Google Analytics), YouTube, DoubleClick, FeedBurner and Grand Central / gizmo5 (Google Voice) have all worked out nicely in retrospect. Others in progress like ITA and AdMob should also be wins.
A lot of their smaller buys were for staff, so it's much harder to say if those panned out.